Focus: You know where you can stick these

James Dyson has left a vacuum at the top of the Design Museum. Chairmen should not resign. It is rather as if the Queen were to emigrate, and this to be interpreted as a broad-brush condemnation of the country from its titular head. Dyson's high-pressure cyclonic departure has been one of the week's more colourful entertainments, a stylish spat among peacocks (and hens). Of course, it shows some baroque egos in a harsh light, but more interestingly still, it exposes the fugitive meaning of "design".

The Design Museum is the most thoughtful monument remaining from the sometimes meretricious design decade of the Eighties, although its spirit is much older still. It was conceived as an energetic successor to the Design Council, whose own origins go back to a Board of Trade initiative in the Thirties.

But by the late Seventies the Design Council had lost authority and was a fatigued and withered arm of state, putting on shows about innovations in drop-forge casting. By the same time, Terence Conran had made enough money out of another, brighter and tighter version of design - the one he sold in Habitat - to pluck me from the obscurity of a provincial university, a Robin to his Batman, to do his works. Conran generously funded, and I directed, a series of successful design promotions that led to me being the founding director of this unique museum, which Mrs Thatcher opened in 1989. It was in sync with the spirit of the age.

Conran had great style, passion and entrepreneurial genius. He was the first Englishman to make a fortune out of the modern design most of his countrymen found unpalatable. He dislikes the word, but his chummy relationship with the media, plus a genuine love of fine things, presented us with "lifestyle". Very pleasantly so.

James Dyson comes from another strand of English life: the inventor-engineer - and is an unusually successful one by English standards. Like Conran, Dyson does not always resist the temptations of self-mythologising. But each is a person of strong beliefs and real achievements. The world is a better place because of what they have done. They deserve respect and attention.

Unfortunately, Alice Rawsthorn, the current Design Museum director, does not agree. Rawsthorn's appointment three years ago followed a couple of lacklustre incumbents in whose tenure the museum lost its sense of direction and ate money. She has found a new direction, but it is not the one the museum was founded to follow. A very professional woman from designer hell - she talks of an admiration for Prada without apparent irony - Rawsthorn is a driven individual of some power and little charm. Undistracted by marriage, she is a teetotaller. Although she does, the editor of an architectural magazine told me in awed tones, "have a table at the Ivy".

This is significant. The Ivy is not a restaurant where you go to eat. It is a celebrity petting zoo. If you find Tom Cruise interesting, you will love it. True sophisticates avoid the Ivy. When Terence Conran and I had lunch last week to go over all of this, we chose an obscure tapas bar in Southwark.

The Design Museum dispute is notionally over Rawsthorn's exhibition about Constance Spry, a flower arranger. Conran thinks this is crass because Spry represents the artless bourgeois mediocrity he made it his life's purpose to eradicate. Dyson thinks it ridiculous because flower arranging debases his interpretation of design as a problem-solving activity, based in technology. But the dispute is really about what "design" actually means. To Conran, design is about making ordinary life more pleasurable for more people. To Dyson, design is about invention and manufacturing. To Rawsthorn, design is à la mode. She chases names, follows fads, worries about if it's in or out. She has a reporter's sensibility, not an investigator's. She reads Wallpaper* and seems to take it seriously. She even writes for it. If she cares anything for the industrial and technological aspects of design, it is not apparent. I doubt that Rawsthorn knows how to calculate brake mean efficiency pressure or that she can describe a radius of gyration.

The great thing about design is what an enormous subject it is: aerospace engineers and hairdressers have a lot in common. Raymond Loewy, the pioneer designer, spoke of his work ranging "from a lipstick to a steamship".

And there is a lot of unexplored middle ground between Dyson's industry and Spry's chrysanthemums. Dyson is wrong if he wants to restrict the Design Museum to titanium lock-nuts, injection-moulding techniques and venturi effects. But Rawsthorn is much more wrong to be in thrall to the momentary gusts of fashion and the clamour of celebrity at the expense of research, knowledge and belief. True, her interest in fads and labels reflects the enlarging nature of "design", but her programme lacks balance and she appears to lack judgement, insight and empathy.

Particularly the last. Terence Conran has sunk perhaps £20m into the Design Museum and is rewarded with a director who pays little attention to what he thinks. The museum can only recover credibility with the broad programme and intellectual depth it presently lacks. The problem with fashion, as Coco Chanel knew, is that it goes out of fashion. I asked one of London's leading art directors what he thought of the Design Museum today. He said: "Why go there?"

Still, I know it was fundamentally a good idea ... it was mine.